1. Field of the Invention
The present invention is related to a voltage regulator circuit and particularly to a power transistor chip with built-in enhancement mode metal oxide semiconductor field effect transistor (enhancement mode MOSFET) and an AC/DC voltage converter using the power transistor chip.
2. Brief Description of the Related Art
Due to the semiconductor technology being developed progressively, the digital products such as the computer and the peripherals thereof are capable of being upgraded continuously. The fast change of the manufacturing process for the semiconductor results in a variety of demands for the power source of the integrated circuit (IC) employed in the computer and the peripherals thereof. Hence, various combinations of voltage regulators using such as the boost converter and the buck converter to meet the need of different power sources of the integrated circuit become one of the most important factors to offer versatile digital products. The AC/DC voltage converter is widely taken as the primary stage circuit of the voltage regulator circuit because it is capable of converting the AC power input to the needed steady direct power output.
Referring to FIG. 1, a circuit diagram of the conventional AC/DC voltage converter is illustrated. The AC/DC voltage converter 10 includes a bridge type rectifying circuit 11, a power transistor chip 12, a pulse width modulation (PWM) circuit 13, a start-up circuit 14, a transformer circuit 15, a filtering and feedback circuit 16 and a working power circuit 17. The pulse width modulation circuit 13 produces the modulated PWM signal to control and output the direct power output Vo according to the magnitude of the feedback voltage of the direct power output Vo. However, the pulse width modulation circuit 13 usually is driven by a low voltage direct power and there is no direct power available for operating the pulse width modulation circuit 13 at the time of the AC/DC voltage converter 10 initiating the work thereof. Therefore, it is necessary to use the start-up circuit 14 and the working power circuit 17 to supply the successive power needed by the pulse width modulation circuit 13.
When the AC/DC voltage converter 10 initiates the work thereof, the output end of the bridge type rectifying circuit 11 sends a rippling direct power to enter the pulse width modulation circuit 13 via the resistance in the start-up circuit 14 for operating the pulse width modulation circuit 13. Then, the pulse width modulation circuit 13 performs the normal job thereof afterward according to the magnitude of the feedback voltage of the output direct power Vo and produces the modulated PWM signal such that the time duration of ON and OFF of the power transistor can be controlled for outputting a steady direct power Vo. The working power circuit 17, which is connected to the transformer circuit 15, supplies more steady working power for the pulse width modulation circuit 13 performs the job thereof much steadily after the AC/DC voltage converter 10 is started up and outputs the steady direct power Vo.
Although the preceding way allows the AC/DC voltage converter to work normally, the start-up circuit 14 keeps in a state of supplying the power to the pulse width modulation circuit 13 unnecessarily. In order to improve the deficiency, the depletion metal oxide semiconductor field effect transistors (Depletion MOSFET) 221, 331 are employed instead to output a start-up signal st via the pulse width modulation circuit 23 or 33 to turn off the operation of the depletion metal oxide semiconductor field effect transistors 221, 331 respectively for conserving the power consumption.
The difference between FIG. 2 and FIG. 3 is in that the depletion metal oxide semiconductor field effect transistors 221, 331 are integrated in the power transistor chip 22 and the pulse width modulation circuit 33 respectively for starting up the AC/DC voltage converter. However, the manufacturing process of the chip becomes more complicated due to the additional process of the channel (N channel or P channel) of the depletion metal oxide semiconductor field effect transistors 221, 331.
Besides, although the preceding different integrations of AC/DC voltage converter circuit such as U.S. Pat. Nos. 6,972,971, 5,014,178 and 5,285,369 are able to comply with different design needs to integrate the start-up circuit, the pulse width modulation circuit and the power transistor, and etc. into a system chip with single chip circuit or two chip circuits, it is incapable of offering the brown-out detection for the pulse width modulation circuit.